Process For Reducing The Arsenic Content Of Gaseous Hydrocarbon Streams By Use Of Supported Lead Oxide

Abstract

A process for reducing the arsenic content of a gaseous hydrocarbon stream by contacting the stream with a sorbent comprising an oxide of lead dispersed upon a supporting material.

Description

This invention relates to the removal of arsenic from gaseous streams and more particularly to a process for reducing the arsenic content of a gaseous hydrocarbon stream by the use of an oxide of lead.

BACKGROUND OF THE INVENTION

Catalytic cracking is one of the principal methods for refining petroleum fractions to recover more valuable hydrocarbon products such as gasoline. The unit in which the cracking operation takes place generally employs a fluidized bed and thus is termed a fluid catalytic cracking (FCC) unit. A variety of lower boiling products in gaseous form are discharged from the FCC unit and these are usually further treated to recover separate hydrocarbon fractions, e.g. ethylene. This further treatment of FCC vapors may, as in the case of the hydrogenation of acetylene, involve the use of a noble metal catalyst. As is well known, nobel metal catalysts are rapidly deactivated by feedstock impurities such as arsenic. It thus becomes desirable to reduce the arsenic content of the FCC gases to the lowest possible level before subjecting them to further treatment.

It should be noted that the exact form in which arsenic is present in FCC gases is difficult to determine. It is known, however, that FCC gases in which arsenic can be detected cause the aforesaid deleterious effects upon a noble metal catalyst. Although it is believed that a major portion of the arsenic contained in the gases is present as arsine (AsH.sub.3), the term "arsenic" as used herein is intended to include arsenic in any combined gaseous form.

DESCRIPTION OF THE PRIOR ART

It is well known that arsenic in gaseous form is a highly toxic substance. Workers in the gas mask art have suggested the use of activated charcoal impregnated with a metal or metal oxide such as lead or lead oxide as a material through which air (or other oxygen-containing gases) may be passed for the removal of arsenic. Exemplary of this proposal is U. S. Pat. No. 1,520,437.

It has been found more recently that the presence of arsenic in gasolines which are treated by contact with a noble metal containing catalyst causes a permanent deactivation of the catalyst. Suggestions have been made in the art to pretreat petroleum fractions to remove arsenic by use of a wide range of materials such as a lignite-based activated carbon (U. S. Pat. No. 3,542,669); silica gel impregnated with sulfuric acid (U. S. Pat. No. 3,093,574); aluminum silicate (U. S. Pat. No. 2,939,833); or a salt of a metal not higher than copper in the electromotive series of metals (U. S. Pat. No. 2,781,297). Since lead is above copper in the electromotive series of metals, the last-mentioned U.S. Pat. No. (2,781,297) would appear to discourage the use of lead in removing arsenic from petroleum feedstreams. Moreover, the art also suggests that lead is an impurity which can contaminate the reforming catalysts employed in petroleum fraction conversion processes; see U. S. Pat. Nos. 2,769,770 and 3,093,574.

SUMMARY OF THE INVENTION

It has been discovered that a suitably dispersed material comprising an oxide of lead will directly remove arsenic from gaseous hydrocarbon streams. For the purposes of this application the lead oxide will be termed a "sorbent," although that term is not intended to suggest that the arsenic removal is accomplished by physical adsorption. While not wishing to be bound by any particular theory, it is believed that some chemical reaction is involved between the arsenic and the sorbent wherein lead-arsenic compounds such as Pb.sub.3 As.sub.2 O.sub.6 may be formed. At a minimum, it is believed that the removal of arsenic is accomplished by chemisorption; that is, the arsenic forms bonds with the surface atoms of the sorbent that are of comparable strength with ordinary chemical bonds and stronger than the bonds formed in physical adsorption.

Surprisingly, the dispersed sorbent will withstand an acceptable loading of arsenic before breakthrough when the arsenic is present in a gaseous light hydrocarbon stream containing both olefins and water vapor. While many materials will function to remove arsenic from admixture with inert gases such as argon and such materials remain active for reasonable loadings of arsenic, most of these materials fail quickly in the removal of arsenic from light hydrocarbon gases such as the gases obtained from FCC units or refinery olefin streams such as streams consisting essentially of ethylene or propylene. In this context, the term "break-through" means the passage of arsenic beyond or downstream of the substance intended to remove it and is usually expressed as a percentage of the arsenic not removed in relation to the arsenic content of the charge stock.

The present invention provides a process for reducing the arsenic content of a gaseous hydrocarbon feedstream which comprises contacting said feedstream with a sorbent dispersed upon a supporting material, said sorbent comprising an oxide of lead. The invention further provides that the supporting material is preferably selected from a high surface area refractory metal oxide or mixtures of refractory metal oxides and most preferably a high surface area alumina. It is further provided that the hydrocarbons in the feedstream have from one to five carbon atoms per molecule with minor amounts of about two percent or less of higher carbon atom molecules such as C.sub.6. Preferably, the hydrocarbons in the feedstream have from one to three carbon atoms with minor amounts of about 10 percent or less of hydrocarbons having from four to six carbon atoms. The feedstream normally includes olefins and water vapor. Preferably, the feedstream is substantially free of sulfur compounds. The arsenic content of the feedstream is generally in excess of 20 ppb and following contact with the sorbent the arsenic content of the feedstream is reduced to less than 20 ppb, preferably less than 10 ppb and more preferably less than 2 ppb. In this application the term "ppb" means "parts per billion" and "ppm" means "parts per million," and such parts are parts by volume unless otherwise indicated. Preferably, the present invention provides that the feedstream is contacted with the sorbent at a temperature in the range of 50.degree. to 400.degree.F. and more preferably in the range of 80.degree. to 250.degree.F.

DETAILED DESCRIPTION OF THE INVENTION

The charge stock for treatment in accordance with the invention is a gaseous hydrocarbon feedstream wherein the hydrocarbons preferably have from one to three carbon atoms per molecule and which feedstream contains aresenic as an impurity, typically in an amount from about 20 parts per billion (ppb) to about 200 parts per million (ppm) or more. Particularly preferred for treatment by the process of the invention are those light hydrocarbon gases obtained by the catalytic cracking of heavier petroleum hydrocarbons such as gas oils for producing primarily gasoline. These light gases from the FCC unit have been found to contain small concentrations of arsenic even though arsine, for example, is known to decompose at about 450.degree.F. and the temperatures in the FCC unit are known to reach over 900.degree.F. There is probably insufficient contact time in an FCC unit to decompose the arsine, or perhaps the arsine decomposes and reforms on cooling.

Preferably, the charge stock is free of sulfur compounds such as H.sub.2 S, since sulfur compounds appear to seriously interfere with the removal of arsines from gaseous hydrocarbon charge stocks. That is, the process of the invention will operate in the presence of sulfur compounds, but the loading of the supported lead oxide before breakthrough will be seriously impaired.

The manner of removing sulfur compounds from the charge stock may be by any of the methods well known in the art. Such methods include, for example, the use of liquid solutions of amines or the use of caustic solutions, e.g., sodium hydroxide solution.

The process of the invention will now be further described by reference to the attached FIGURE. Referring to the FIGURE, the petroleum charge for catalytic cracking enters through line 2 into FCC unit 4 where it is converted under usual catalytic cracking conditions to a variety of lower boiling products, including gasoline type products. Gasoline is removed from FCC unit 4 through line 6. The other gaseous products of the cracking process, which products are of primary concern here, are removed from FCC unit 4 through line 8 and enter an absorber section 10. Absorber section 10 normally consists of several component units (not shown) such as an amine absorber, a knock-out drum to remove any entrained liquids from the gaseous products; and a heater to insure that the gases remain in the vapor phase. The FCC gases exiting from the heater unit of absorber section 10 have the typical composition shown in the following Table I:

TABLE I

Component Vol. % Nitrogen 9.5 Hydrogen 9.8 Methane 29.7 Ethylene 9.7 Ethane 12.6 Propylene 15.5 Propane 6.8 Butenes 0.5 Butanes 1.9 Pentenes 0.4 Pentanes 0.5 Hexanes 0.1 Carbon Monoxide 2.9

The FCC absorber gases are usually at a temperature from 80.degree. to 150.degree.F., more usually from 100.degree. to 125.degree.F., and at a pressure from 250 to 400 psig, more usually at a pressure from 290 to 360 psig. The increased pressures are those normally employed in the FCC unit and are used to propel the gases through the various units in the recovery train. The absorber gases leave the absorber section 10 through line 12 and pass into arsenic removal unit 14.

The function of arsenic removal unit 14 is to reduce the concentration of arsenic in the FCC absorber gases from a concentration in excess of 20 ppb to a concentration at the outlet of less than 20 ppb. The concentration of arsenic in the FCC absorber gases is usually on the order of 50 to 750 ppb but can be as high as 20 ppm or more. Preferably, the arsenic content of the gases is lowered to less than 10 ppb and more preferably to less than 2 ppb by arsenic removal unit 14.

The type of solid material employed in arsenic removal unit 14 is an important feature of the invention and will be discussed in detail hereinbelow. Suffice it to say here that the material comprises an oxide of lead well dispersed upon a suitable support having a high surface area.

The temperatures to be employed in arsenic removal unit 14 can suitably be from 50.degree. to 400.degree.F., are usually from 80.degree. to 250.degree.F., and are preferably from 100.degree. to 200.degree.F. Temperatures below 50.degree.F. are undesirable because of the increased cost and the decreased activity of the sorbent at those levels. Temperatures above the stated range are undesirable due to the increased expense of operating the process. Apart from economic considerations, however, high temperature levels, which would otherwise promote hydrogenation of olefins present in the feed stream when certain other sorbent materials are employed, are not of concern in the process of the present invention since lead oxide is not a hydrogenation catalyst. Higher operating temperatures do have the advantage in the process of the invention of prolonging the life of the lead oxide sorbent before regeneration is required.

The pressure to be employed in arsenic removal unit 14 is suitably atmospheric pressure or below, to 1000 psig or more. FCC units typically operate to produce product gases, as noted above, at pressures from about 250 to 350 psig. The process of the present invention operates well at atmospheric pressure, but since it is expensive to depressure the FCC absorber gases and repressure the final products for transport through pipelines, it is desirable to operate the arsenic removal process at increased pressure of, say, 250 to 350 psig. A limitation on the maximum operating pressure is, however, the effect of pressure on promoting undesirable side reactions such as the polymerization of any olefins which may be present in the feedstream. The gaseous volume hourly space velocity (GVHSV) at standard conditions of temperature and pressure can suitably be from 1,000 to 36,000 v/v/hr and is usually from 2,000 to 10,000 v/v/hr. The product is removed from the arsenic removal unit 14 through line 16.

Light hydrocarbon gases such as ethane and propane are fed through line 18 into pyrolysis furnace 20 for the purpose of cracking the ethane and propane to produce ethylene. After removal of liquid products (not shown) from pyrolysis furnace 20, the gaseous products are passed through line 22 where they are combined with the products in line 16 from the arsenic removal unit 14.

The combined gases in line 24 enter system 26 which consists of a number of units, not individually shown, for the purpose of drying and recovering various hydrocarbon fractions. A C.sub.3 fraction, for example, can be removed through line 28 and a C.sub.4 fraction through line 30. The stream of most present interest and of greatest volume is the C.sub.2 stream containing small amounts of acetylene, which stream is shown in the FIGURE as being removed from system 26 through line 32 and which pases into an acetylene converter 34. The acetylene content is produced in the pyrolysis furnace 20. Hydrogen enters acetylene converter 35 by means of line 35.

Acetylene converter 34 may contain a catalyst which is sensitive to poisoning by even minute quantities of arsenic, and thus it is one of the main objectives of the present invention to protect the catalyst in the acetylene converter 34 from permanent deactivation by arsenic. Catalysts which are particularly susceptible to arsenic poisoning are those containing the noble metals such as platinum and palladium. Hydrogenation conditions are, of course, employed in acetylene converter 34, and such conditions are well known to workers skilled in the art. The C.sub.2 stream, substantially free of acetylene, is then taken from acetylene converter 34 through line 36 to a distillation zone 38 where ethylene is removed through line 40 and heavier products may be suitably removed through line 42. The heavier products may be recycled as feed to pyrolysis furnace 20 if desired.

It should be noted here that the aresenic removal unit 14 could have been positioned immediately before the acetylene converter 26, if desired. Similarly, the same benefits would accrue for any arsenic-susceptible catalysts used in the hydrogenation of the propadiene in the C.sub.3 stream from line 28.

PREPARATION OF DISPERSED SORBENT

The sorbent employed in the process of the invention is most easily converted to a high surface area form by dispersion onto a suitable high surface area support. The manner of dispersing the sorbent on the supports is not critical and may be accomplished by means well known in the art. One method is described in detail in Example 1 below. Briefly, the technique involves the deposition of lead from a solution, preferably aqueous, of a suitable lead salt such as lead nitrate followed by calcining in the presence of air to produce a sorbent comprising lead oxide. The lead salt which is employed must be one which will decompose to the desired lead oxide form on calcining or which can be oxidized to the desired lead oxide form under conditions which will not impair the desired surface area characteristics of the support.

The amount of lead dispersed on the support is suitably from 5 to 50 weight percent and preferably from 10 to 30 weight percent of the total sorbent plus support.

Suitable high surface area supports are those well known in the art as catalyst supports. Examples of suitable supporting materials are the usual porous naturally occurring or synthetically prepared high surface area, i.e., over about 50 m.sup.2 /g, refractory metal oxides well known in the art as catalyst supports, e.g., alumina, silica, boria, thoria, magnesia or mixtures thereof. Preferably the supporting material is one of the partially dehydrated forms of alumina. More preferably, the alumina is one having a surface area in excess of 50 m.sup.2 /g, preferably a surface area of 150 to 350 m.sup.2 /g. Suitable forms of the higher surface area aluminas and their methods of preparation are described in the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Volume 2, pages 41 et seq. Other suitable supports include clays, zeolites and crystalline silica aluminas.

EXAMPLE 1

The purpose of this example is to describe one preparation of a lead oxide material supported by high surface area alumina. An aqueous solution of lead nitrate was prepared by adding 837.21 g. of Pb (NO.sub.3).sub.2 (Mallinckrodt Analytical Reagent Grade) to distilled water to give a final folume of 1670 ml. The weight of this solution was 2322 g. and its specific gravity was 1.3904 g/cc. It contained the equivalent of 22.55 percent Pb.

A one-step incipient impregnation of the alumina was carried out by adding, with stirring, the Pb(NO.sub.3).sub.2 solution to 2055 g. of 1/16-inch extrudates of a suitable alumina which had previously been heated to 1,000.degree.F. over a period of 6 hours and held at 1,000.degree.F. for 10 hours. The incipient wetness absorptivity of the alumina was 0.8127 ml/g of alumina. The wet material was dried with occasional stirring for 12 hours at 250.degree.F. The dry material was then calcined by raising the temperature to 1,000.degree.F. over a period of 6 hours and holding at 1,000.degree.F. for 9 hours. The final calcined composition analyzed 20 weight percent lead calculated as the metal. The compacted density was 0.804 g/cc and it had a nitrogen B.E.T. surface area of 160 m.sup.2 /g and a nitrogen pore volume of 0.471.

The final sorbent was off-white in appearance. X-ray analysis of the sorbent showed the presence of some crystalline lead sulfate, which is white. There is a small amount of sulfate associated with the alumina base (1.08 percent sulfur), and this probably accounts for the presence of the lead sulfate. A similar preparation using a very low sulfur base (0.08 percent) showed the presence by X-ray diffraction analysis of the complex 4PbO.sup.. PbSO.sub.4 which is also white. The lead nitrate from which the sorbent was prepared is known to decompose at conditions far less severe than the calcination conditions. Thus, while it is not certain, due to the complex chemistry of lead oxides, it is believed the lead is primarily present as PbO or some combination of PbO with lead sulfate due to the light color of the finished sorbent. Other forms of lead oxide such as PbO.sub.2, Pb.sub.2 O.sub.3 and Pb.sub.2 O are highly colored.

Technical grade solutions may be employed in the foregoing procedure. The solutions are normally added at room temperature but elevated temperatures may be utilized. The alumina used in this preparation had a nitrogen B.E.T. surface area of 282 m.sup.2 /g and a pore volume of 0.63 cc/g.

ARSENIC REMOVAL

Several runs were made under varying conditions to illustrate the present invention. The results of these runs are presented in Examples 2-9 summarized in Table II below. The procedures employed for all tests were identical and were as follows: Gaseous charge stocks were prepared by mixing a sufficient amount of a blend of 2,000 ppm AsH.sub.3 in nitrogen (supplied by Matheson Gas Co.) with one of the following diluent gases to obtain a charge stock having the designated ppm of AsH.sub.3 as shown in Table II below.

Diluent Gas No. 1. An ethylene stream having the following approximate analysis:

Component Vol. % Ethylene 65.0 Ethane 35.0 Acetylene 0.5

Diluent Gas No. 2. A pure hydrocarbon blend having the following approximate analysis:

Component Vol. % Propylene 15 Ethane 12 Ethylene 10 Methane 30 Hydrogen 15 Nitrogen 18 Total 100

Diluent Gas No. 3. A commercial FCC absorber gas of the following analysis:

Typical Component Vol. % Range Vol. % Carbon Monoxide 1.6 0.2-3.4 Hydrogen 7.9 9-12 Nitrogen 9.8 6-10 Methane 30.0 27-33 Ethylene 9.8 9-11 Ethane 12.4 10-13.0 Propylene 17.2 15-18.0 Propane 7.6 7-15 Butenes 0.4 0-1.0 Isobutane 1.3 1-2.0 n-Butane 0.1 0-1.0 C.sub.5 1.7 0-3 C.sub.6 0.2 0-1 Total 100.0 Arsenic 450 ppb 50-750 ppb Hydrogen Sulfide 1 ppm(wt) 0-2 ppm Carbonyl Sulfide 3.4 ppm(wt) 0-5 ppm

Diluent gases Nos. 2 and 3 were passed through a water bubbler to saturate them with water vapor at ambient temperature prior to adding arsine. Diluent gas No. 1 was not saturated with water.

The reactor containing the supported sorbent consisted of a 3/8 inch I.D. stainless steel cylinder, with a 1/8 inch O.D. thermowell extending along its axis. The reactor was suitably heated. The temperature at the center of the supported sorbent material was measured by means of an iron-constantan thermocouple inserted into the thermowell. The test gas was introduced at the bottom of the reactor, passing through an approximately 6-inch-long bed of quartz chips which served to preheat the gas stream.

The sorbent was dispersed on activated alumina in accordance with the procedures set forth in Example I. The bed of supported sorbent within the reactor was approximately 4 to 8 inches in length and consisted of 5-10 cc. of material sized to 20-40 mesh. In all cases, the weight percent of lead compared with the total weight of support material and sorbent was 20 percent.

The arsine not removed by passage through the bed of supported sorbent was scrubbed from the effluent gas stream by a pyridine solution containing 0.50 g. silver diethyldithiocarbamate (Fisher Certified Reagent) per 100 ml. pyridine. This silver salt combines with the arsine to form a highly colored complex, permitting colorimetric monitoring of the total arsine breakthrough accumulation. Small samples were periodically drawn from the arsine scrubber and the optical transmittance at 540 mm. wavelength was measured with a Bausch and Lomb Spectronic 70 spectrophotometer. This optical transmittance was then plotted as a function of time. The numerical derivative of this curve was calculated to determine the rate of arsine breakthrough. The percent breakthrough figures given in Table II below represent the percentage of the arsenic not removed in relation to the arsenic content of the charge stock. ##SPC1##

Referring to Table II, the run for Example 5 demonstrates the telling effect of having H.sub.2 S in the charge stock. The runs for Examples 7-9 show that regeneration at higher temperatures increased the loadings of arsenic achieved.

Another series of runs was made wherein a slipstream of a commercial FCC absorber gas without the addition of added amounts of AsH.sub.3 was passed through a 1" in diameter by 4-foot-long bed of a lead oxide sorbent prepared as described in Example 1. The commercial FCC absorber gas had a composition within the range as shown above for Diluent Gas No. 3. Again, there were no added amounts of AsH.sub.3. The results are presented in the Example given below.

EXAMPLE 10

In the run for this Example, the bed was operated at 80.degree. to 105.degree.F. and a pressure of 260--280 psig and a GVHSV of 9,000 for 8 days at which time a small breakthrough of arsenic was noted and the GVHSV was reduced to 4,500. The run was continued for a total of 2,300 hours when, again, a small breakthrough of arsenic was noted. Within 48 more hours, the breakthrough was 10 percent, and this increased to 20 percent after a total of 3,000 hours. It was calculated that about 1.7 weight percent arsenic was present on the sorbent at initial breakthrough (2,300 hours) and about 2.2 weight percent arsenic after 3,000 hours.

Yet another series of runs was made wherein a slipstream of a commercial ethylene concentrate (having the approximate analysis of Diluent Gas No. 1 above) and containing about 40--400 ppb of AsH.sub.3 was passed through a 1" in diameter by 4-foot-long bed of a lead oxide sorbent prepared as described in Example 1. The results are given in Example 11 below.

EXAMPLE 11

In the run for this Example the bed was operated at 120.degree. to 150.degree.F. and a pressure of 230-300 psig and a GVHSV of 9,000 for a total time of 3,230 hours without breakthrough, at which point the run was discontinued. Estimated calculation indicated the loading of arsenic to be 0.65 weight percent.

In each of Examples 10 and 11, a fresh batch of sorbent was employed.

Resort may be had to such variations and modifications as fall within the spirit of the invention and the scope of the appended claims.

Claims

We claim:

1. A process for reducing the arsenic content of a gaseous hydrocarbon containing feedstream which comprises contacting said feedstream with a sorbent dispersed upon a supporting material, said sorbent comprising lead oxide.

2. A process as recited in claim 1 wherein said supporting material is a high surface area alumina.

3. A process according to claim 2 wherein said lead oxide is PbO.

4. A process as recited in claim 2 wherein said hydrocarbons in said feedstream have from one to five carbon atoms per molecule.

5. A process as recited in claim 4 wherein said hydrocarbon feedstream includes oelfins and water vapor.

6. A process as recited in claim 5 wherein said contacting takes place at a temperature in the range of 50.degree.F. to 400.degree.F.

7. A process for reducing the arsenic content of a gaseous hydrocarbon containing feedstream containing arsenic in amounts in excess of 20 ppb, which process comprises contacting said feedstream with a sorbent dispersed on a supporting material, said sorbent comprising lead oxide.

8. A process as recited in claim 7 wherein said supporting material is a high surface area alumina.

9. A process as recited in claim 8 wherein said hydrocarbons in said feedstream have from one to five carbon atoms per molecule.

10. A process according to claim 9 wherein said lead oxide is PbO.

11. A process as recited in claim 9 wherein said hydrocarbon feedstream includes olefins and water vapor.

12. A process as recited in claim 11 wherein said contacting takes place at a temperature in the range of 80.degree. to 250.degree.F.

13. A process as recited in claim 12 wherein said arsenic content of said feedstream after contacting is less than 10 ppb.

14. A process as recited in claim 10 wherein said arsenic content of said feedstream after treating is less than 2 ppb.

15. A process in accordance with claim 7 wherein the gaseous hydrocarbon feedstream is a commercial FCC absorber gas.

16. A process in accordance with claim 15 wherein the absorber gas has a composition comprising:

17. A process for reducing the arsenic content of a gaseous hydrocarbon containing feedstream containing arsenic in amounts in excess of 20 ppb, which process comprises contacting said feedstream with a sorbent dispersed on a supporting material, having a surface area in excess of 50 m.sup.2 /g, said sorbent consisting of lead oxide.

18. A process according to claim 17 wherein said supporting material is alumina having a surface area from 150-350 m.sup.2 /g.